The present invention relates to an integrated high voltage generating system used in high voltage pulse generating circuit of semiconductor memory device or the like.
In an EEPROM or the like using MNOS transistors, in order to write and erase data in and from MNOS transistors, it is necessary to apply a high voltage of about 15 V to the gate electrode. Usually, semiconductor device such as EEPROM, a power supply of about 5 V is used as an operating power source. Accordingly, a high voltage pulse generating circuit is incorporated into an integrated circuit of EEPROM, and the input supply voltage of about 5 V is boosted by this high voltage pulse generating circuit to a high voltage of 15 to 20 V.
Such high voltage pulse generating circuit is composed of, as disclosed in the Japanese patent application No. 62-122931 or the U.S. Pat. No. 4,839,787, a boosting charge pump connecting multiple stages of pump made of a diode element and a capacitor, high voltage generating system for applying the high voltage supplied from the charge pump into the gate electrodes of MNOS transistors of EEPROM, and others.
FIG. 3 shows a one-stage portion (unit circuit) of a conventional integrated high voltage generating system used in application of high voltage to gate electrodes of MNOS transistors of EEPROM.
In FIG. 3, a first supply voltage of about 5 V, for example, is applied to a first power source input terminal 4. At a second power source input terminal 5, a second supply voltage Vp of about 15 V, which is boosted, for example, by a charge pump as disclosed in the Japanese patent application No. 62-122931 or the U.S. Pat. No. 4,839,787, is applied. The first supply voltage is fed to a node A through a diode-connected first MOS transistor (hereinafter called first transistor) 1. The second supply voltage Vp is fed to the node A which is the input node of the first supply voltage through a node C and a third MOS transistor (hereinafter called third transistor) 3. On the other hand, at the node A, clock pulses having a high frequency of several MHz and an amplitude VDD nearly equal to the first supply voltage (5 V in this case) are supplied from a clock pulse source 6 by way of a first capacitor 7 as a coupling capacity. The voltage of the node A supplied to a node B which is the output node through a diode-connected second MOS transistor (hereinafter called second transistor). Between the node B and the reference potential point, a second capacitor 8 is connected as a load capacity, and the voltage at the upper end of the second capacitor 8 is delivered from an output terminal 9 as an output voltage after boosting. Meanwhile, the gate electrode of the third transistor 3 is connected to the node B.
The operation of the integrated high voltage generating system in FIG. 3 is explained below.
The basic operating principle of the MOS transistor is known as follows. Supposing the voltages of the gate, drain and source electrodes of a MOS transistor to be VG, VD and VS, respectively, when VD.gtoreq.VG, it follows that EQU VS=VG-(VT+.DELTA.VT) (1)
where VT is the threshold voltage of the MOS transistor, and .DELTA.VT is the back bias effect.
In the following explanation, the sum of VT+.DELTA.VT is assumed to be 2 V.
The first supply voltage (5 V) applied to the first power source terminal 4 is supplied to the node A through the first transistor 1, and hence the potential VA at the node A is obtained as follows from equation (1): ##EQU1## However, since clock pulses of amplitude VDD (5 V) are applied to the node A from the clock pulse source 6, the potential VA at the node A is raised to about 8 V at the moment the clock pulses rise up. This 8 V potential VA is supplied to the node B through the second transistor 2, and hence the potential VB at the node B is obtained as follows from equation (1): ##EQU2##
At the rise moment of a clock pulse, the third transistor 3 is turned off, but, as mentioned above, along with the rise of a clock pulse, the second transistor 2 is turned on, and along with the fall of a clock pulse, the second transistor 2 is turned off. At this time, the potential VB at the node B is raised to about 6 V by the electric charge accumulated in the second capacitor 8, and therefore the third transistor 3 is turned on. As a result, the second supply voltage Vp (15 V) applied to the second power source input terminal 5 is supplied to the node A through the third transistor 3. At this time, from equation (1), the potential VA at the node A becomes as follows: ##EQU3##
As clear from the comparison between equation (1) and equation (4), the potential VA at the node A was 3 V before the rise of a clock pulse, and it is boosted to 4 V when the clock pulse once rises and then falls. In this period, the potential VB at the node B is boosted to 6 V, and this voltage is delivered from the output terminal 9 as the output voltage.
Thus, in the integrated high voltage generating system in FIG. 3, the second and third transistors 2, 3 are alternately switched repeatedly in synchronism with the clock pulses, and the potential VA at the node A is boosted stepwise sequentially, and in response the potential VB at the node B is also boosted stepwise sequentially. As a result, a stepwise boosted output voltage is obtained from the output terminal 9. The output voltage from the output terminal 9 is finally boosted to the same potential as the second supply voltage (15 V), and this voltage is applied to the gate electrodes of the MNOS transistors composing the EEPROM (not shown).
As evident from the description herein, in FIG. 3, a charge pump is composed of the first, second and third transistors 1, 2 and 3, and the second capacitor 8, and the potential at the node B (output node) is boosted stepwise sequentially from the first supply voltage (5 V) to the second supply voltage Vp (15 V) in synchronism with the clock pulses applied through the first capacitor 7.
Incidentally, when a power source having a sufficiently large current sink capacity is connected to the second power source input terminal 5 in FIG. 3, the potential VC of the node C is almost fixed at the value of the second supply voltage Vp. However, when a charge pump as disclosed in the Japanese patent application No. 62-122931 or the U.S. Pat. No. 4,839,787 is connected to the second power source input terminal 5, the following problem may occur.
The capacitors composing the stages of such charge pump have usually several 10 pF, and are capable of passing electric currents of only several microamperes, and therefore the current sink capacity is small. Besides, when the number of bits of the EEPROM increases, the leak current also increases, which also causes to lower the current sink capacity of the charge pump. When a power source with such small current sink capacity is connected to the second power source input terminal 5 in FIG. 3, the potential VC of the node C is swept away by the clock pulses supplied through the first capacitor 7.
Meanwhile, supposing the pulse amplitude of the clock pulse source 6 to be VDD, the fluctuation amplitude of potential VC of node C to be .DELTA.V, the capacity of the first capacitor 7 to be C1, and the floating capacity of the node C to be Cs, the equivalent circuit in FIG. 3 becomes as shown in FIG. 4. In FIG. 4, assuming that the electric charge Q occurring in the first capacity is entirely transferred to the node C, it follows that EQU Q=C1(VDD-.DELTA.V)=Cs.multidot..DELTA.V (5)
From equation (5), we obtain ##EQU4##
As stated above, in the integrated high voltage generating system shown in FIG. 3, the peak value of the amplitude of the potential Vc of the node C is held in the second capacitor 8 by the switching action of the second and third transistors 2, 3, and becomes the potential of the node B (output node). The potential at this node B is delivered from the output terminal 9 as the output voltage. In this process, the pulse amplitude waveform of the clock pulses is superposed on the potential VC of the node C, and a potential fluctuation of .DELTA.V occurs. Accordingly, the output voltage taken out of the output terminal 9 must be, in principle, a constant voltage determined by the second supply voltage Vp, but the output voltage actually obtained is the voltage determined by (Vp+.DELTA.V), that is, .DELTA.V higher than the second supply voltage Vp.
This problem further leads to the following problem when a plurality of integrated high voltage generating system shown in FIG. 3 are connected as shown in FIG. 5.
In FIG. 5, the boosting blocks l to n correspond to the integrated high voltage generating system shown in FIG. 3, individually. Supposing the fluctuation amplitude of the potential VC of the node C to be .DELTA.Vn (.DELTA.Vn denotes the fluctuation amplitude when n boosting blocks are connected), the capacity value of the first capacitors 7l to 7n to be C11, C12, . . . , C1n, and the floating capacity of the node C to be Csn (Csn denotes the floating capacity when n stages are connected), it follows from equation (5) that ##EQU5## From equation (7), we obtain ##EQU6##
From equation (8), it is known that when C1M increases, the value of the left side becomes small, and hence the value of the right side also becomes small. In other words, .DELTA.Vn approaches VDD. That is, the fluctuation amplitude .DELTA.Vn of the potential VC of the node C approaches the amplitude VDD of the clock pulses.
In this way, when a plurality of boosting blocks are connected as shown in FIG. 5, the number of first capacitors 7l to 7n connected to the common clock pulse source 6 increases, and therefore the fluctuation amplitude .DELTA.Vn of the node C increases. As a result, the output voltage obtained from the output terminals 9l to 9n of the boosting blocks l to n changes to Vp+.DELTA.Vn. As stated above, since .DELTA.Vn is the value determined by the number of stages of the boosting blocks, when there is one boosting block (that is, in the composition shown in FIG. 3), the output voltage taken out of the output terminal 9 is the voltage determined by Vp+.DELTA.V1 as shown in FIG. 6 (a), and when there are two boosting blocks, the output voltage of two boosting blocks is the voltage determined by Vp+.DELTA.V2 as shown in FIG. 6 (b), and when there are n boosting blocks, the output voltage of all boosting blocks is the voltage determined by Vp+.DELTA.Vn as shown in FIG. 6 (c). Hence, as stated above, the maximum value of .DELTA.Vn is a value close to the amplitude VDD of the clock pulses.
When using the integrated high voltage generating system as shown in FIG. 5 as the high voltage pulse generating circuit of EEPROM, it is necessary to assemble plural integrated high voltage generating systems in a different number of stages depending on the number of bits. However, since the output voltage of the plural integrated high voltage generating systems varies depending on the number of stages of the boosting blocks, the second supply voltage Vp supplied from the charge pump (not shown) cannot be correctly transmitted to the gate electrodes of the MNOS transistors (not shown).
It is hence a first object of the invention to prevent an integrated high voltage generating system capable of eliminating the influences of voltage fluctuations due to clock pulses.
It is a second object of the invention to present an integrated high voltage generating system capable of preventing voltage fluctuations due to clock pulses from appearing on the output voltages of boosting blocks even when a plurality of boosting blocks are connected and clock pulses are applied to plural boosting blocks from a common clock pulse source through individual capacitors.